It is very common practice to use liquid storage tanks for such fluids as gasoline, diesel fuel, and oil. These storage tanks are usually filled through openings that extend upwardly from the top of the tanks. Such openings usually have relatively small diameters that are sized to receive a dispensing tube, which dispensing tube is connected to a tank truck or other supply source. In most cases, these storage tanks do not have gauges and the operator filling the storage tank has no visual access to the internal space of the storage tank, or any other reliable way of determining when the storage tank is approaching a full level. As a consequent of these factors, overfilling of the storage tanks is a common occurrence. Such overfilling of a storage tank may result in spillage of the tank contents, damage to the tank or filling equipment, or injury to persons. Concerns over spillage of the tank contents is particularly acute when the contents are flammable, toxic, or otherwise potentially dangerous or environmentally hazardous. The problems associated with overflow spillage have become so widespread that many local governments now require some liquid storage tanks to be filled only with equipment that automatically shuts off the flow of the flow of liquid to the storage tank when the tank is full.
One way to automatically shutoff the flow of liquid to a full storage tank that has been employed in the prior art is to place a shutoff valve in the dispensing line used for supplying liquid to the storage tank. These types of shutoff valves usually employ floats that are moved by a rising liquid level in the tanks. The movement of the floats is, in turn, used to move a closure element in the shutoff valve from an open position to a closed position that stops or significantly reduces the flow of fluid through the drop tube. Examples of such shutoff valves in drop tubes are illustrated in U.S. Pat. No. 4,986,320 to Kesterman et. al. and U.S. Pat. No. 4,667,71 to Draft.
The shutoff valves described above perform very satisfactorily for underground storage tanks where the tanks are filled only under gravity pressure, which pressure is typically in the range of 4 to 5 psi. However, such shutoff valves are unsuitable when above ground storage tanks are used, or when, for any other reason, pumping pressure (not merely gravity pressure) is used to drive the fluid being introduced into the storage tanks. A typical pump used for driving liquid to an above ground storage tank delivers liquid at a pressure of approximately 50-60 psi, more than ten times the pressure encountered in gravity feed systems. Among other difficulties involved when such pumping pressure are used is the increased potential for line shock in the supply hose and related components. When a valve is closed rapidly, the flow through the valve is reduced sharply, and the pressure on the upstream side of the valve increased correspondingly. This action results in a shock wave of high pressure that is propagated upstream, applying high pressure pulses to the supply line and all of its related components. If the valve is closed sufficiently rapidly, the shock wave resulting from that closure may damage, or even rupture, the supply line and result in leakage. Because of the increased pressure conditions associated with the filling of above ground storage tanks, both from operating the valve components under the increased dynamic pressures of the fluid during the filling process and the increased line shock resulting from valve closure, shutoff valves used for underground storage tanks have generally proved to be unsuitable for use in above-ground tanks.
There have been at least two commercialized attempts in the prior art to solve the problems associated with shutoff valves for above ground storage tanks and the increased pressures involved therewith. In one of these attempts, a drop tube is provided with a shutoff valve located at its lower terminus. The valve includes circumferential outlet ports. A float is slidably fitted on the outside surface of the drop tube, and the float is interconnected to a restrictor sleeve for common movement therewith on the drop tube. As increased fluid levels occur in the storage tank, the float, and thus the restrictor sleeve, is urged upwardly. This upward movement of the restrictor sleeve positions it over the circumferential outlet ports of valve, restricting further filling of the storage tank. Among other deficiencies, this type of shutoff valve relies solely on gravity for returning the float to an open position once liquid in the storage tank is depleted. Thus, it is proned to reset failure. Moreover, this method of shutting off fluid flow inherently requires the shutoff valve to be near the bottom of the drop tube, making it prone to clog-up from solid matter that typically settles in the bottom of storage tanks. Still further, such valves do not completely shut off the flow of liquid into the storage tank.
In another less than fully successful prior art attempt to solve the above described problems, a plug valve is disposed in a drop tube. The plug valve includes a rotatable plug member that is connected by rods to a float. As the fluid level in the storage tank is increased, the float causes the connecting rods, and thus the rotatable plug, to rotate. This type of shutoff valve arrangement has the disadvantage of significantly reducing fluid flow when partially closed, and thus significantly slowing down the filling last portion of the filling process. Furthermore, this valve relies only on the weight of the float to return the valve to an open position once fluid in the storage tank is removed.